Highly coherent extreme ultraviolet radiation around the water window region (∼4.4 nm) is generated in a semi-infinitive helium gas cell using infrared pulses of wavelength 1300 nm, energy 2.5 mJ, duration 40 fs, and repetition rate 1 kHz. The pressure-squared dependence of the intensity and the almost-perfect Gaussian profile and low divergence of the high harmonic source indicate a phase-matched generation process. The spatial coherence of the source is studied using Young’s double-slit measurements.